Midseason Disease Decisions

Published: July 23, 2019

Scouting and making management decisions for disease will bring a few challenges this season. Across the state, there is a considerable amount of developmental variability in the corn and soybean acreage due to delayed or re- planting this spring. These differences in growth stage from one field to the next will also mean differing levels of susceptibility to disease during the season. When making management decisions, keep in mind how the factors of the disease triangle will change from field to field, especially in regards to susceptibility to disease based on host growth stage.

With warm, humid conditions pushing us into the midseason, some common foliar diseases have started to show in parts of the state. Here's the rundown of the major players and some things to consider.

Corn

Southern rust

Southern rust made its appearance in Missouri on July 13th, right on schedule. In the fields where southern rust has been confirmed, infection levels have been low. However, following last week's tropical storm and with the forecast calling for warm, humid conditions, scouting efforts should be stepped up in the coming weeks. It is important to note that late planted corn that is still in the vegetative growth stages will remain susceptible longer and will need to be monitored closely for southern rust development. Look for orange- to cinnamon-colored rust pustules on the upper leaf surface that are densely clustered together on the leaf. Southern rust can be confused with common rust, a pathogen that is not generally yield limiting, so it is important to confirm the identity of the pathogen prior to a fungicide application.

When making fungicide decisions for southern rust management, think back to the disease triangle and the factors necessary for disease development.

Pathogen factor – Southern rust is in the field

Environment factors – Daytime temps from 75-93°F and 4 or more hours of leaf wetness

Host factor – Susceptible hybrid, growth stage of initial onset

If all three of the above factors are present, a yield benefit can be observed when applying a fungicide through the R3 or Milk stage of development. By R4 or dough stage, there may be a benefit in some cases from a fungicide application and by R5 or dent stage and beyond, a yield benefit is unlikely to occur. The key is to reduce infected leaf area during the critical stages of grain fill and development.

If you suspect southern rust, samples can be sent to the MU Plant Diagnostic Clinic for confirmation. Additionally, you can track southern rust progress in the U.S. at corn.ipmpipe.org.

Gray leaf spot

Another corn disease that has started to develop is gray leaf spot (GLS). Unlike southern rust, the GLS pathogen does not blow into the state but survives on corn residues present at the soil surface. GLS generally begins in the lower canopy and lesions can often takes up to two weeks to develop after initial infection. Initial lesions can be small with irregular margins. As lesions develop on more susceptible hybrids they are restricted by the leaf veins and become their characteristic rectangular shape. Due to the slow development of new lesions, scouting for GLS is a critical first step to understanding if and when to apply fungicides.

Fungicide decisions for GLS management involve several factors to consider:

Pathogen factors – Canopy level of lesions, leaf area affected

Environment factors – Temperatures ranging from 75-85°F and humid weather (<90% relative humidity)

Host factors – Hybrid resistance level, growth stage of initial lesion development

The slow development of new lesions and timing of initial lesion development plays an important role in infection potential. Additionally, utilizing resistant hybrids can slow lesion development and size which can substantially decrease disease and affected leaf area. For these reasons, all factors must be considered when making fungicide decisions for GLS control.

Soybean

Frogeye leaf spot

As fields move into the early reproductive stages of development, the characteristic round, tan lesions of frogeye leaf spot (FLS) are beginning to develop. The pathogen of FLS survives on residues present at the soil surface or can be blown into a field after the infection cycle begins. New lesions develop on younger leaf tissues, so it is best to begin scouting for developing lesions on recently emerged or newly emerging trifoliates. Now is the time to step-up scouting efforts to assess FLS presence.

Before making fungicide decisions, consider the following:

Pathogen factors – FLS lesions developing in the field, residues present

Environment factors – Forecast for warm, humid weather

Host factors – Varietal resistance level, leaf age, developmental stage at onset

Resistant varieties available for managing FLS can provide sufficient control of this disease in most years. In Missouri and surrounding states, isolates of the FLS pathogen resistant to the strobilurin class of fungicides have been recovered. For this reason, if fungicides are considered it is recommended to utilize multiple modes of action and use full label rates. Remember to follow all label instructions and restrictions when applying fungicides.

More information about major corn and soybean diseases in the U.S. can be found through the Crop Protection Network and foliar fungicide efficacy can be found on the 2019 Corn Foliar Fungicide Efficacy and 2019 Soybean Foliar Fungicide Efficacy tables.